1. Field of the Invention:
The present invention relates to a display apparatus of an active matrix driving system.
2. Description of the Related Art:
A display apparatus of an active matrix driving system comprises a plurality of scanning signal lines, a plurality of data signal lines, and pixels provided to the respective intersections of the scanning signal lines and the data signal lines. Each pixel includes, as shown in FIG. 29, a switching element 101 and a pixel capacitance C.sub.P. The pixel capacitance C.sub.P includes two electrodes and a display medium therebetween, one of which is connected to a common line 104. The switching element 101 is made of a TFT (thin film transistor). A data signal line 102 and the other of the electrodes of the pixel capacitance C.sub.P are connected to each other via the drain and source terminals of the TFT. The gate terminal of the TFT in the switching element 101 is connected to a scanning signal line 103. Therefore, when the scanning signal line 103 is activated, the switching element 101 is turned on, thereby transferring a pixel data on the data signal line 102 to the pixel capacitance C.sub.P as an electric charge. In this manner, an image based on the pixel data is displayed. Even after the switching element 101 is turned off, an electric field is applied to the display medium by the electric charge accumulated in the pixel capacitance C.sub.P, and therefore, the displayed image is maintained.
However, a leak resistor R having a comparatively small resistance is actually present in parallel with the pixel capacitance C.sub.P as is shown in FIG. 30. Therefore, the electric charge accumulated in the pixel capacitance C.sub.P leaks through the leak resistor R as a leak current. In addition, since the capacity of the pixel capacitance C.sub.P is generally as small as 0.1 pF or less, the amount of the electric charge accumulated in the pixel capacitance C.sub.P is small as shown in FIG. 31. When this small amount of the electric charge leaks as a leak current, the voltage is largely decreased. As a result, the electric charge in the pixel capacitance C.sub.P is gradually lost by the leak current and the voltage is largely decreased during the data-holding period between writing periods, when the switching element 101 is on and the electric charge based on The pixel data is accumulated in the pixel capacitance C.sub.P. Such decrease in the voltage of the pixel capacitance C.sub.P during the data-holding period causes a flicker, that is, visual variation in the displayed image, thereby degrading the display quality.
A liquid crystal display apparatus is generally driven by a so-called alternating driving method, in which The electric field whose polarity is alternately changed is applied to the pixel capacitance C.sub.P so as to prevent the degradation of the liquid crystal. Also in this alternating driving method, the voltage of the pixel capacitance C.sub.P is decreased by a leak current during the data-holding periods of a positive field and a negative field as shown in FIG. 32, resulting in the same problem of the degradation of the display quality.
As a method for solving this problem, a sample and hold circuit as shown in FIG. 33 is provided to each pixel. This method is disclosed in Japanese Laid-Open Patent Publication No. 3-77922. In this method, when the switching element 101 is turned on, the pixel data is first supplied to a holding capacitance C.sub.H (for sampling), and when the switching element 101 is turned off, the electric charge based on the pixel data is held in the holding capacitance C.sub.H (for holding). Then, a transistor 105 supplies an electric charge to the pixel capacitance C.sub.P through a source line 106 in accordance with the voltage of the holding capacitance C.sub.H. In this circuit, a capacitance with a small leak current can be used as the holding capacitance C.sub.H because it is merely a capacitance element. The transistor 105 is a N-channel MOSFET to which the voltage of the holding capacitance C.sub.H is input. This transistor 105 together with the pixel capacitance C.sub.P as a load forms a voltage follower circuit as a buffer amplifier. Therefore, the transistor 105 can supply a positive electric charge in accordance with the voltage of the holding capacitance C.sub.H to the pixel capacitance C.sub.P without losing the electric charge of the holding capacitance C.sub.H. The pixel capacitance C.sub.P is charged so as to have a voltage lower than the voltage of the holding capacitance C.sub.H by the threshold voltage of the transistor 105. Therefore, in the pixel shown in FIG. 33, the supplied pixel data is thoroughly held by the holding capacitance C.sub.H, and the electric charge based on this pixel data can be continuously supplied to the pixel capacitance C.sub.P by the switching transistor 105. Therefore, the voltage in the pixel capacitance C.sub.P is prevented from reducing during the data holding period, and the degradation in the display quality is avoided.
However, in the circuit as is shown in FIG. 33, the buffer amplifier using the transistor 105 can be operated merely unidirectionally, i.e., it only supplies a positive electric charge to the pixel capacitance C.sub.P. Therefore, when a pixel data with a smaller amount of an electric charge than that of the previously supplied pixel data is supplied, the pixel capacitance C.sub.P disadvantageously continues to hold the previous electric charge.
Moreover, in a liquid crystal display apparatus of the alternating driving, the buffer amplifier only comprising such a unidirectional transistor 105 can not supply a negative electric charge to the pixel capacitance C.sub.P. Therefore, such a buffer amplifier can not be used in the negative field. Thus, it is impossible to provide a practical display apparatus.